1. Field of the Invention
The present invention relates to an optical fiber for use in a 1.3 .mu.m band optical fiber amplifier to be used in optical communication systems, and more specifically, to a rare earth metal ion-doped fluoride-based optical fiber having a high gain coefficient for optical amplifiers.
2. Description of the Prior Art
In recent years, studies have been energetically performed on optical fiber amplifiers which involve doping of the core of an optical fiber with rare earth metal ions, especially Er.sup.3+ ions, to induce stimulated emission as a result of transitions within the 4 f shell, so that they may be applied to 1.5 .mu.m optical communication systems. Rare earth metal ion-doped optical fiber amplifiers are high in gain and have gain characteristics not dependent on polarization of a beam. They also have a low noise index and broad-band wavelength characteristics. Therefore, their application to optical communication system is very attractive.
The 1.3 .mu.m band, in which wavelength dispersion in a quartz-based optical fiber becomes zero, is an important wavelength band for optical communication along with the 1.5 .mu.m band. Research into optical fiber amplifiers operating at 1.3 .mu.m has been carried out using Nd.sup.3+ ion doped silica-based optical fibers or fluoride optical fibers .
In both optical fibers, however, the excited state absorption of Nd.sup.3+ ions is great at a wavelength of 1.31 .mu.m which is used in optical communication. Therefore, no amplification has been confirmed as described, for example, in W. J. Miniscalco, L. J. Andrews, B. A. Thompson, R. S. Qiuinby, L. J. B. Vacha and M. G. Drexhage, "Electron. Lett." (vol. 24, 1988, p. 28) , or Y. Miyajima, T. Komukai, Y. Sugawa and Y. Katsuyama, "Technical Digest Optical Fiber Communication Conference '90 San Francisco" (1990, PD16) .
Under these circumstances, there is a keen demand for optical fiber amplifiers which have an amplifying effect at 1.31 .mu.m. One of the candidates is an optical fiber amplifier using an optical fiber comprising a ZrF.sub.4 -based fluoride glass as a host material which is doped with Pr.sup.3+ as laser-activating ions, as proposed in Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takanashi, E. Snitzer and G. H. Sigel "Technical Digest Optical Fiber Communication Conference '91 San Diego" (1991, PD2) . This optical fiber amplifier utilizes the stimulated emission of the .sup.1 G.sub.4 -.sup.3 H.sub.5 transition of Pr.sup.3+ ions, as seen from the energy level diagram of Pr.sup.3+ ions in FIG. 1. That is, light emission at the 1.3 .mu.m band is due to the transition from .sup.1 G.sub.4 to .sup.3 H.sub.5. As shown in FIG. 1, the lower level .sup.3 H.sub.5 is higher than the ground state .sup.3 H.sub.4, thus forming a four level system. The central wavelength of emission is 1.322 .mu.m. The wavelength used for pumping is 1.017 .mu.m which causes excitation from the ground level .sup.3 H.sub.4 directly to the upper level .sup.1 G.sub.4 .
The above-mentioned optical fiber amplifier, however, is defective in that the energy difference between the .sup.1 G.sub.4 level and the .sup.3 F.sub.4 level is as small as about 3,000 cm.sup.-1 . In detail, the phonon energy of the ZrF.sub.4 -based fluoride glass as the host material is 500 cm.sup.-1. Hence, phonon relaxation from the .sup.1 G.sub.4 level to the .sup.3 F.sub.4 level easily occurs, and the quantum efficiency of the .sup.1 G.sub.4 -.sup.3 H.sub.5 transition is as low as 3% in the ZrF.sub.4 -based fluoride glass, thus making the degree of amplification per unit pump power, i.e. gain coefficient, remain at about 0.2 dB/mW (Y. Ohishi, T. Kanamori, J. Temmyo, M. Wada, M. Yamada, M. Shimizu, K. Yoshino, H. Hanafusa, M. Horiguchi and S. Takahashi, Electronics Letters, vol. 27, no. 22, pp. 1995-1996, 1991) .
Thus, a high pump power of 25 mW is required for obtaining a gain of, for example, 5 dB.
Since the output power of a laser diode has its own limitations, however, an increase in the gain coefficient of a rare earth metal ion-doped optical fiber for an optical amplifier is necessary in order to obtain a sufficiently high gain for practical use by pumping the laser diode.
One measure for realizing the high efficiency of an optical fiber amplifier is to incorporate PbF.sub.2 into zirconium glass, thereby increasing the refractive index of the core. With a conventional method in which BaF.sub.2 is substituted by PbF.sub.2, however, the difference (.DELTA.T) between the crystallization temperature (Tx) and the glass transition temperature (Tg) of the core glass (.DELTA.T is generally used as an indicator of the heat stability of glass) decreases with the increase in the PbF.sub.2 content. Hence, the core crystallizes upon heating during optical fiber fabrication, resulting in an increased transmission loss, thereby decreasing the effective gain. Therefore, the relative refractive index difference .DELTA..sub.n between the core and the cladding of the fluoride optical fiber is at most 1.2%.